Solid Phase Synthesis
Solid phase synthesis is an established and effective method for the preparation of peptides, and offers advantages over conventional solution phase chemistry in terms of purification and simplicity (Atherton E, Sheppard R C, Solid Phase Peptide Synthesis: A Practical Approach; IRL Press at Oxford University Press: Oxford, 1989). Solid phase synthesis may also be used for the preparation of non-peptide molecules (Leznoff C C, Acc. Chem. Res., 1978, 11, 327-333) and recently there has been considerable interest in the application of this methodology to the synthesis of combinatorial libraries for biologically active lead compound optimisation and discovery (Moos W H et al., Annu. Rep. Med. Chem., 1993, 28, 315-324).
Solid phase synthesis requires an appropriate solid substrate which carries a plurality of functional groups to which the first reactive entity in the proposed synthesis may be covalently coupled, and from which the desired molecule may be cleaved after assembly. The solid substrate should be compatible with the solvents and reaction conditions that are to be used in the peptide or non-peptide synthesis.
The final step in solid phase synthesis is the cleavage of the covalent bond between the desired peptide or non-peptide molecule and the linker. It is desirable that the conditions for the cleavage are orthogonal to those used during the reactions employed for the synthesis of the peptide or non-peptide on the solid support such that inadvertent cleavage does not occur during the synthesis. Furthermore, the conditions for cleavage should be relatively mild such that they do not result in degradation of the desired peptide or non-peptide. Solid substrates which present hydroxyl groups as the points of attachment for the first stage of the synthesis are commonly used, for example substrates which present hydroxyl groups as derivatives of benzyl alcohol, the peptide or non-peptide being attached as a benzyl ester and cleaved by hydrolysis, transesterification or aminolysis to release the peptide or non-peptide as a carboxylic acid, carboxylate ester or as a carboxamide. Also used are substrates which present amino groups, for example as derivatives of diphenylmethylamine, the peptide or non-peptide being attached as a carboxamide and cleaved by hydrolysis to release the peptide or non-peptide as a carboxamide. Substitution of such linkers by a nitro group can enable the photolytic cleavage of the peptides or non-peptides from the residue of the solid substrate.